[Hardlec23]

I have never set up the Microsoft Companion. I have now downloaded and installed the windows version. 
Once I link the tablet and laptop, do I need the companion?

[Skip]

You don't 'need' the companion, but it makes uploading much easier uploading/downloading to a tablet. It's not used at all for MSP on a windows install, you just use 'local' as the source.

[Hardlec23]

I have had MSP on my tablet for some time.  I love it.  My only quibble is that I can't use the full version of Musescore on my Tablet, I can only do this on my laptop.

now I have MSP on both my laptop and my tablet.  I'd like to "compose" on MSP and Musescore on the laptop, but use the Tablet for rehearsals and performing.

[Skip]

If you have MSP on each device, essentially yes.
I have MSP on my laptop and several tablets and my phone. I find or prep all my music on the lap top and copy it to a file labeled 'For tablet' [I also have a file 'On Tablet' I use to back up this stuff]. Then I use the [Connect to] Companion via wifi to load the songs in the 'for tablet' onto my primary tablet. I do all my annotations on that tablet so I make 2 backup's of the primary tablet, 1 on the laptop [using the Companion] in a folder labeled 'MSP backups' and one on the primary tablet. I use the backup on the laptop [local source] to update [restore] the new material. I use the laptop to restore [update] the other devices [minus the primary] to the updated material via wifi using the 'Connect to Companion' option [located in the overflow drop down, 3 stacked dots]. 

Since you are using only one tablet you would just do the backup then restore the laptop/tablet using the backup on the laptop. Bottom line, load 2 [or more] times, once for each device. I think you could also use the 'sync library' function, but I've never tried it.

Hope that's not too confusing.